Mixers of the generic kind, which are also referred to as annular-layer mixers, are widely known, for example from U.S. Pat. No. 3,734,471. Special measures, in the form of mixing tools which acutely taper in the direction of the container wall, have been taken in the case of such annular-layer mixers, in particular for the application of glue to wood fibres, in order to separate the fibres which, after the application of glue, tend to form clusters, as is known from U.S. Pat. No. 4,006,887. These measures have, however, not had the desired effect, i.e. to separate the clusters.
In order to assist the separation of clusters, the residence time of the wetted materials in the mixer was extended by providing, at the outlet, adjustable or controlled adjustable outlet valves, as a result of which an adjustable or controllable backpressure was exerted on the material, in order thus to subject it for a longer period of time to the separating action of the specific tools. This resulted in considerable caking of the material in the outlet region, which was also not desirable.